1. The Field of the Invention
This invention relates generally to optical spectroscopy devices and techniques. In particular, the present invention relates to optical filtration and spatial positioning devices for use in the analysis of multiple channels of a light signal.
2. The Relevant Technology
Spectroscopy is a well known technique that involves the production and investigation of the spectral content of polychromatic light. Such forms of light are made up of numerous different wavelengths, and spectroscopy allows for the analysis of these individual wavelengths. This form of analysis has broad applications in fields such as chemistry, biology and telecommunications. For example, a common application utilizes a device known as a spectroscope, which sends a light signal through a sample and then disperses the individual wavelengths of the emitted light signal onto a grid. The characteristics of the sample composition can then be identified depending on which wavelengths are actually emitted. The spectral information can be used to identify the sample in much the same way that a fingerprint can be used to identify an individual in that no two elements emit the same spectra.
Another important application of spectroscopy is in the field of optical communications. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that can interfere with electrical signals. Light also provides a more secure signal because it does not emanate the type of high frequency components often experienced with wire-based electrical signals. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.
Another advantage in using light as a transmission medium is that multiple wavelength components of light can be transmitted through a single communication path such as an optical fiber. This process is commonly referred to as wavelength division multiplexing (WDM), where the bandwidth of the communication medium is increased by the number of independent wavelength channels used. Spectroscopy techniques can be used to investigate and verify the presence of these different wavelength channels by separating light signals into constituent wavelength sets, channel groups, or separate wavelengths.
One problem associated with the use of spectroscopy techniquesxe2x80x94especially in optical communicationsxe2x80x94is the difficulty in dispersing the individual light signal wavelengths in a manner that can be efficiently and accurately detected at a high resolution. This is especially the case in dense wavelength division multiplexing (DWDM) applications where the individual wavelength communication channels are closely spaced to achieve higher channel density and total channel number in a single communication line. For example, most spectroscopy devices use a prism or a diffraction grating device as a dispersion member to separate wavelength components. However, these devices separate the wavelengths in a linear manner, such that they are dispersed along a particular line. Thus, to detect the dispersed wavelengths, detectors must be placed along a line in a corresponding plane. The number of required detectors is proportional to the number of detected wavelengths and desired resolution. Thus, to detect a broad range of wavelengths, a very long line of detectors must be employed, which takes up a relatively large amount of space and increases the overall cost and complexity of the optical communications system.
Another approach is to use a mechanical device to aim the different wavelengths at a single detector for correspondingly different time periods. For example, a rotating reflective diffraction grating can be used to direct the individual wavelengths to a single detector location for a specific time period. Again, this approach has several drawbacks. While it reduces the number of detectors required, it utilizes devices with moving parts and having relatively high mechanical complexity, thereby increasing cost and reducing reliability. Moreover, the approach can be inefficient. For example, if a large number of wavelengths are involved, the approach introduces a relatively large time delay, an especially undesirable characteristic in any communications application.
Yet another problem encountered when utilizing such spectral analysis techniques is related to the accurate detection of the particular channels in question. In particular, if the physical dispersion of individual wavelengths is too narrow or the sampling detectors elements too few, there is a risk of focusing unwanted wavelength(s) onto the same detector elements as the desired wavelength. This would obviously create noise and distort the information contained within the desired channel. More expensive high dispersion diffraction gratings can be obtained to disperse the wavelengths into a broader area and therefore onto a sufficient number of detector elements per wavelength channel to allow high resolution and accurate detection. However, this solution requires a large number of linear detector elements, additional space, and more complex and expensive focusing optics. Therefore, most spectroscopy applications must balance the need for higher resolution with the expense and size ramifications of using a broader dispersion member.
These and other problems are addressed by embodiments of the present invention, which relates to a holographic demultiplexor or optical spectroscopy system that separates and images the constituent wavelengths of a light signal onto a two-dimensional detection device. Moreover, the holographic demultiplexor reflects and spatially positions selected wavelengths onto a two-dimensional detector array, as opposed to an unreasonably long linear detector array. Embodiments of the present invention are thus particularly suitable for spectral wavelength separation, as would be done in typical spectroscopy systems and applications.
In general, presently preferred embodiments include a volume hologram. The volume hologram includes a hologram for each wavelength or set of wavelengths that is included in the light signal. Each hologram reflects a particular range of wavelengths or a specific wavelength. In one embodiment, each wavelength is reflected at a particular orientation. This configuration allows the optical element to precisely reflect individual wavelengths that are dispersed from a dispersing member, such as a prism or a diffraction grating, towards specific points on a corresponding detector. The volume hologram can be used to provide two dimensions of spatial dispersion, thus facilitating the use of a two dimensional detector array.
The advantages of the present invention include the ability to individually filter and reflect optical wavelengths, or channels, to specific optical detector locations. By filtering and reflecting individual channels in this manner, embodiments of the present invention minimize the physical space otherwise needed for linearly detecting a broad range of wavelengths. In addition, the present invention maximizes resolution by efficiently utilizing a large number of detector elements, available in a two-dimensional detector array, for sampling individual wavelength channels.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by the practice of the invention. The features and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out in the appended claims. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.